A version of Hashtable supporting
concurrency for both retrievals and updates:

Retrievals

Retrievals may overlap updates. (This is the same policy as
ConcurrentReaderHashMap.) Successful retrievals using get(key) and
containsKey(key) usually run without locking. Unsuccessful
retrievals (i.e., when the key is not present) do involve brief
synchronization (locking). Because retrieval operations can
ordinarily overlap with update operations (i.e., put, remove, and
their derivatives), retrievals can only be guaranteed to return the
results of the most recently completed operations holding
upon their onset. Retrieval operations may or may not return
results reflecting in-progress writing operations. However, the
retrieval operations do always return consistent results -- either
those holding before any single modification or after it, but never
a nonsense result. For aggregate operations such as putAll and
clear, concurrent reads may reflect insertion or removal of only
some entries.

Iterators and Enumerations (i.e., those returned by
keySet().iterator(), entrySet().iterator(), values().iterator(),
keys(), and elements()) return elements reflecting the state of the
hash table at some point at or since the creation of the
iterator/enumeration. They will return at most one instance of
each element (via next()/nextElement()), but might or might not
reflect puts and removes that have been processed since they were
created. They do not throw ConcurrentModificationException.
However, these iterators are designed to be used by only one
thread at a time. Passing an iterator across multiple threads may
lead to unpredictable results if the table is being concurrently
modified.

Updates

This class supports a hard-wired preset concurrency
level of 32. This allows a maximum of 32 put and/or remove
operations to proceed concurrently. This level is an upper bound on
concurrency, not a guarantee, since it interacts with how
well-strewn elements are across bins of the table. (The preset
value in part reflects the fact that even on large multiprocessors,
factors other than synchronization tend to be bottlenecks when more
than 32 threads concurrently attempt updates.)
Additionally, operations triggering internal resizing and clearing
do not execute concurrently with any operation.

There is NOT any support for locking the entire table to
prevent updates. This makes it imposssible, for example, to
add an element only if it is not already present, since another
thread may be in the process of doing the same thing.
If you need such capabilities, consider instead using the
ConcurrentReaderHashMap class.

Because of how concurrency control is split up, the
size() and isEmpty() methods require accumulations across 32
control segments, and so might be slightly slower than you expect.

This class may be used as a direct replacement for
java.util.Hashtable in any application that does not rely
on the ability to lock the entire table to prevent updates.
As of this writing, it performs much faster than Hashtable in
typical multi-threaded applications with multiple readers and writers.
Like Hashtable but unlike java.util.HashMap,
this class does NOT allow null to be used as a key or
value.

Implementation note: A slightly
faster implementation of this class will be possible once planned
Java Memory Model revisions are in place.

table

CONCURRENCY_LEVEL

protected static final int CONCURRENCY_LEVEL

The number of concurrency control segments.
The value can be at most 32 since ints are used
as bitsets over segments. Emprically, it doesn't
seem to pay to decrease it either, so the value should be at least 32.
In other words, do not redefine this :-)

loadFactor

threshold

votesForResize

Number of segments voting for resize. The table is
doubled when 1/4 of the segments reach threshold.
Volatile but updated without synch since this is just a heuristic.

keySet

protected transient java.util.Set keySet

entrySet

protected transient java.util.Set entrySet

values

protected transient java.util.Collection values

Constructor Detail

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
float loadFactor)

Constructs a new, empty map with the specified initial
capacity and the specified load factor.

Parameters:

initialCapacity - the initial capacity.
The actual initial capacity is rounded to the nearest power of two.

loadFactor - the load factor threshold, used to control resizing.
This value is used in an approximate way: When at least
a quarter of the segments of the table reach per-segment threshold, or
one of the segments itself exceeds overall threshold,
the table is doubled.
This will on average cause resizing when the table-wide
load factor is slightly less than the threshold. If you'd like
to avoid resizing, you can set this to a ridiculously large
value.

Throws:

java.lang.IllegalArgumentException - if the load factor is nonpositive.

ConcurrentHashMap

ConcurrentHashMap

public ConcurrentHashMap(java.util.Map t)

Constructs a new map with the same mappings as the given map. The
map is created with a capacity of twice the number of mappings in
the given map or 32 (whichever is greater), and a default load factor.

Method Detail

bitcount

protected static int bitcount(int w)

Return the number of set bits in w.
For a derivation of this algorithm, see
"Algorithms and data structures with applications to
graphics and geometry", by Jurg Nievergelt and Klaus Hinrichs,
Prentice Hall, 1993.
See also notes by Torsten Sillke at
http://www.mathematik.uni-bielefeld.de/~sillke/PROBLEMS/bitcount

hash

protected static int hash(java.lang.Object x)

Return hash code for Object x. Since we are using power-of-two
tables, it is worth the effort to improve hashcode via
the same multiplicative scheme as used in IdentityHashMap.

put

Maps the specified key to the specified
value in this table. Neither the key nor the
value can be null. (Note that this policy is
the same as for java.util.Hashtable, but unlike java.util.HashMap,
which does accept nulls as valid keys and values.)

The value can be retrieved by calling the get method
with a key that is equal to the original key.

Specified by:

put in interface java.util.Map

Overrides:

put in class java.util.AbstractMap

Parameters:

key - the table key.

value - the value.

Returns:

the previous value of the specified key in this table,
or null if it did not have one.

putAll

Copies all of the mappings from the specified map to this one.
These mappings replace any mappings that this map had for any of the
keys currently in the specified Map.

Overrides:

putAll in class java.util.AbstractMap

Parameters:

t - Mappings to be stored in this map.

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface java.util.Map

Overrides:

clear in class java.util.AbstractMap

clone

public java.lang.Object clone()

Returns a shallow copy of this
ConcurrentHashMap instance: the keys and
values themselves are not cloned.

Overrides:

clone in class java.util.AbstractMap

Returns:

a shallow copy of this map.

keySet

public java.util.Set keySet()

Returns a set view of the keys contained in this map. The set is
backed by the map, so changes to the map are reflected in the set, and
vice-versa. The set supports element removal, which removes the
corresponding mapping from this map, via the Iterator.remove,
Set.remove, removeAll, retainAll, and
clear operations. It does not support the add or
addAll operations.

Specified by:

keySet in interface java.util.Map

Overrides:

keySet in class java.util.AbstractMap

Returns:

a set view of the keys contained in this map.

values

public java.util.Collection values()

Returns a collection view of the values contained in this map. The
collection is backed by the map, so changes to the map are reflected in
the collection, and vice-versa. The collection supports element
removal, which removes the corresponding mapping from this map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll, and clear operations.
It does not support the add or addAll operations.

Specified by:

values in interface java.util.Map

Overrides:

values in class java.util.AbstractMap

Returns:

a collection view of the values contained in this map.

entrySet

public java.util.Set entrySet()

Returns a collection view of the mappings contained in this map. Each
element in the returned collection is a Map.Entry. The
collection is backed by the map, so changes to the map are reflected in
the collection, and vice-versa. The collection supports element
removal, which removes the corresponding mapping from the map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll, and clear operations.
It does not support the add or addAll operations.